Methods for producing sponges of gelatin, collagens, fibrin, poly (glycolic acid) (PGA) and poly(lactic acid) (PLA), etc. have been known for some time. While many techniques exist for producing foams for biomaterial applications, however, most involve the use of organic solvents and some are prohibitively expensive to employ. One common technique is solvent casting followed by particulate leaching. The polymer is first dissolved in organic solvent, and then it is mixed with a solid “porogen” such as table salt. The solvent is evaporated, leaving the salt crystals cast in the polymer. Next, the composite is leached with water to remove the salt, leaving the porous material. Another common class of techniques is phase separation/emulsification. A foam may be produced containing polymer dissolved in organic solvent then beat into a foam with water. The foam is then frozen and freeze-dried to remove the solvent and water. Techniques based on freeze drying are not well suited to large scale operations. Freeze drying is a very expensive method of removing water, due to the expensive equipment required, the slow rate of dehydration and high energy consumption.
Conventional methods of drying to produce a foam include air drying, freeze drying, and vacuum drying. Air drying produces pores in a solid or semi-solid material by incorporating a leaving agent, pore-casting, or salt elution. Often this process takes a long time, or is expedited by application of heat. Freeze drying takes a considerable amount of time, and is limited by the space available in the apparatus. It is also expensive due to the equipment required and the energy consumed to effect sublimation. Vacuum drying does not allow control of energy input rate, and thus it is difficult to control pore size or pore wall thickness in the resulting foam.
Cellular solids can also be produced from gels. Gels are widely used in the food industries, and diffusion of solutes into foods is common practice (Rassis et al., 1997). Recently, dried gels have been proposed to serve as carriers for food ingredients such as vitamins and minerals and also drugs after surgery or treatments. Hydrocolloid gels can be derived from polysaccharides, yielding fine textured gels at low polymer concentration, or from proteins using higher polymer concentrations. The production of dried hydrocolloid gels is simple, quick and inexpensive. Control of their physical properties in terms of porosity and mechanical strength would enable their use for a wider range of purposes. They can also be used to control the acoustic response of specific dry food products and have a great potential for future use in countless different fields, from foods and packaging to medicine and medical care, daily commodities, farming and agriculture and the environmental chemical and even electronic industries.
Hydrocolloid gels have a network structure that swells in an appropriate solvent. Swelling of a gel involves an increase of a network pressure that results from elastic extension of the polymeric matrix. When this network pressure becomes relaxed by means of dehydration, shrinkage may take place. During dehydration, the hydrophilic polymer matrix is surrounded by water before drying, and air after drying. These phases may be considered as good and poor solvents, respectively. A poor solvent may favor polymer-polymer interaction, and thus may induce a spontaneous collapse. The collapse is induced by the change in solvent quality during dehydration. Capillary forces have also been considered as one of the reasons for collapse. The end point of the shrinkage or collapse may be the transition from the rubbery to the glassy state of the product. The hydrocolloid gel physics indicates that a drastic increase in rigidity is possible by percolation of filler particulates (Eichler et al., 1997).
When two polymers in the form of macroscopic particles are mixing together, there is a chance of phase separation of the polymeric blend in the dried material. This kind of separation depends on various parameters like individual solubility of the polymers in the solvent used, interaction with substrate surface, method of deposition and method of drying. To avoid these problems, nano-particles of polymers are combined and dried (see Kietzke et al., 2003). They demonstrated that aqueous dispersions containing nano-particles of various polymers could be produced by a “miniemulsion” process. They dissolved the polymers first in suitable solvent then added it to an aqueous solution containing an appropriate surfactant. By applying high shear, a stable emulsion containing small droplets of polymer solution (the so-called mini-emulsion) is obtained.
Hydrocolloid foams and sponges can be produced by freeze dehydration either immediately after their production or after their immersion in different carbohydrate solutions to change their physical and chemical compositions. The resultant dried cellular structures are an interconnected network of pores in a solid structure. Varying the preparation procedures can modify the mechanical properties of these sponges. For example, internal gas bubbles in wet agar gels drastically reduced the mechanical integrity of the dried sponges and affected their porosity. However, the same process in alginate sponges caused only minor mechanical changes (Nussinovitch et al., 1993). Oil included in alginate gels weakens the mechanical strength of the dried sponges, lowers its stress and stiffness at failure as reflected by the deformability modulus, and changes the size distribution and structure of pores of the dried sponges (Nussinovitch and Gershon, 1997). Water plasticization of sponges changes their stress-strain behavior. Vacuum dried gels or those conditioned to water activity 0.33 collapsed by brittle fracture. Sponges conditioned to water activity 0.57 and 0.75 appeared to collapse by elastic buckling (Rassis et al., 1998).
Most gels have a low solid content and have therefore rather low total solids for efficient drying. Hydrocolloid foams and sponges are dry gel products that may be economically feasible, depending on the cost of the drying process involved.
Cellular solids have a low density and low mechanical strength based on the cell wall and the entire cellular structure. Their structure can be classified according to the following characteristics. Flexibility vs. brittleness of cell wall; distribution of cell size in the body of the cellular solid; open vs. closed cells; thickness and shape of the cell wall; and structure uniformity as mentioned on different length scale. The most valued properties of cellular solids are their density, conductivity, Young's modulus, and strength. Cellular solids usually have relative densities of less than 0.3 kg/m3, but they may reach a lower value. Different structures of cellular solids lead to a wide range of such properties and a much greater utility. A low density substance translates to light, stiff, large portable structures that are able to float. Their low thermal conductivity brings about thermal insulation.
There is a need in the art for new and improved methods of producing foams and sponges from hydrocolloids. Further, there is a need for methods of drying biologically active material which allow good control of temperature during the drying process for materials which may be heat labile or heat sensitive.